Distributed acoustic sensing (DAS) based on Rayleigh backscatter is a known technique. The basic principle is that coherent illuminating radiation, typically in the form of one or more pulses of optical radiation, is used to repeatedly interrogate an optical fibre, referred to herein as the sensing fibre.
Consider that the sensing fibre is interrogated by a single pulse of coherent radiation launched into a first end of the sensing fibre. As the pulse propagates along the fibre the phenomenon of Rayleigh scattering from various inherent scatting sites within the optical fibre will result in some small proportion of the interrogating radiation being backscattered toward the first end, where it can be detected. The backscatter signal received back at the first end of the sensing fibre is thus a combination of various signals from different parts of the fibre illuminated by the pulse as it propagates. With coherent interrogating radiation the backscatter signal is thus an interference signal formed from radiation scattered from the various inherent scattering sites. As the scattering sites are effectively randomly distributed throughout the sensing fibre the intensity of the backscatter signal received will exhibit a random variation from one section of the fibre to the next. However, in the absence of any environmental stimulus acting on the fibre, the backscatter signal from a given portion of the sensing fibre will be the same from interrogation to interrogation, assuming the properties of the interrogating radiation are the same for each interrogation.
An environmental disturbance acting on a portion of the sensing fibre that results in an effective change of optical path length for that portion, such as a dynamic strain on the fibre, will however result in a change in the backscatter signal from that portion between interrogations. By monitoring the backscattered radiation received at the first end of the sensing fibre, e.g. using a suitable photodetector, such a change can be detected and used to indicate dynamic disturbances, e.g. incident acoustic waves, acting on the relevant portion of sensing fibre.
In some DAS systems each interrogation comprises launching a single continuous pulse of interrogating radiation. In such systems the backscatter signal is typically processed to look for intensity variations in the backscatter from various longitudinal sensing portions of the optical fibre in order to detect disturbances acting on the sensing fibre. In other systems each interrogation may comprise launching (at least) two spatially separated optical pulses, which may be at different frequencies, and in such systems the processing may look for changes in phase of the measurement signal from a given sensing portion, possibly at a carrier frequency defined by the frequency difference between the pulses.
Location along the sensing fibre is determined based on OTDR (optical time domain reflectometry) techniques, with the backscatter signals being processed in time bins corresponding to backscatter from defined portions of the fibre. This technique relies on the fact that light detected a given time after the interrogating radiation was launched into the sensing fibre must have been scattered from a given position along the length of the sensing fibre. However for this assumption to be correct the backscatter detected must be uniquely associated with a given interrogation, thus a second interrogation (with the same optical properties as a first interrogation) cannot be launched into the fibre until light from the first interrogation has reached the distal end of the fibre and then any backscatter has travelled the entire length of the fibre back toward the detector and has been detected. Were the second interrogation to be launched whilst radiation from the first interrogation was still propagating in the sensing fibre it wouldn't be possible to distinguish backscatter arising from the first interrogation (from relatively far into the fibre) from backscatter from the second interrogation (from nearer the first end of the fibre).
This limits the repetition rate for interrogations to the round trip time in the fibre. The maximum pulse rate, RP, is thus RP=c/2Ln, where c is the speed of light in vacuo, L is the length of the fibre (or, for very long fibres, the threshold distance into the fibre from which no significant backscatter can be expected) and n is the refractive index. For a fibre with a length L of 5 km and a refractive index of about 1.5 the maximum pulse repetition rate, RP, is of the order of 20 kHz. This sets the Nyquist limit for the frequency of acoustic stimuli that can be reliably detected.
It has been proposed to improve the pulse repetition rate by using wavelength divisional multiplexing techniques, e.g. by launching a first interrogation at a first wavelength and then a second interrogation at a second wavelength. As the interrogations use different wavelengths the backscatter from each interrogation can be separately identified and processed, thus allowing radiation from both interrogations to be propagating in the fibre at the same time without introducing any positional ambiguity.
However the use of wavelength division techniques necessitates multiple sources and detectors and adds to the cost and complexity of the interrogator unit.
In addition the spatial resolution achievable by such DAS sensors depends, at least partly, on the pulse duration. For a single pulse DAS sensor the minimum size of a sensing portion is effectively defined by the pulse duration, and thus the spatial length of the pulse in the fibre. At any instant the backscatter received back at the first end of the fibre corresponds to the backscatter from a section of fibre of a length equal to half the length of the pulse in the fibre. Thus it would not be possible to independently sense signals affecting sensing portions of the fibre at a length shorter than this. To provide a better spatial resolution would require shorter pulses, but shorter pulses involve transmitting less optical power into the sensing fibre (the maximum intensity of the pulses is limited by the need to avoid non-linear effects in the fibre). Reduced energy of the interrogating radiation results in reduced sensitivity. Thus for conventional DAS sensors there is a trade-off between sensitivity and spatial resolution.